1. Field of the Invention
The present invention relates generally to microwave circuits having distributed constant lines each formed on a major surface of a substrate, and more particularly, to a microwave circuit suitably made into a monolithic microwave integrated circuit.
2. Description of the Background Art
FIG. 11 is a block diagram of a conventional receiver for SHF (Super High Frequency) band direct satellite broadcast (12 GHz band). This receiver comprises an antenna 50, a high frequency low noise amplifier 51, a band pass filter or an image band-stop filter 52, a mixer 53, a local oscillator 54, an intermediate frequency amplifier 55 and an intermediate frequency output terminal 56. A signal received by the antenna 50 is amplified by the high frequency low noise amplifier 51 and thereafter is inputted to the mixer 53, wherein the signal is mixed with a local oscillator signal Lo from the local oscillator 54 and then converted into a signal of the intermediate frequency. The filter 52 in such circuit structure attenuates a signal in the image frequency band which is the problem in mixing.
The above described conventional circuit of FIG. 11 is formed by a MIC (Microwave IC) circuit having circuit devices such as transistors, diodes or the like disposed on dielectric substrates and connected through microstrip lines. A half-wavelength parallel coupled line filter using microstrip lines is often used as the filter circuit 52.
In recent years, field effect transistors and HEMTs using GaAs and InP have been developed because GaAs and InP have mobility of electrons five or six times as large as that of Si, and furthermore various attempts have been made in many places to make such transistors and matching circuits, bias circuits or the like monolithic. The monolithic IC of this type is referred to as MMIC (Monolithic Microwave IC) which is more compact and has excellent reliability as compared with the conventional MIC circuits. Thus, it is reported that MMIC is applied to such a low noise amplifier and a mixer for receiving SHF band direct satellite broadcast.
A transmission line for use in an MMIC includes a microstrip line, a coplanar line and the like. The microstrip line is formed by forming a main line out of a gold conductor on a major surface of a semiconductor substrate having a conductive ground electrode on its back surface. The coplanar line has ground electrodes and a main line formed on a major surface of a semiconductor substrate.
FIG. 12 shows a GaAs low noise amplifier MMIC for directly receiving a satellite broadcast (12 GHz band) as an example of a conventional microwave circuit having distributed constant lines on a major surface of a semiconductor substrate.
This MMIC comprises an input terminal 13 to which microwave is inputted, FET1 for amplifying a microwave signal with low noise, an input main line 5 connected between the input terminal 13 and a gate 2 of the FETl, an input matching stub 6 with one end thereof connected to the input terminal 13 and a termination grounded through a capacitor 9a, a bias terminal 11 for biasing a gate of the FET1 connected to a termination of the input matching stub 6, an outputting main line 7 connected to a drain 3 of the FETl, an outputting stub 8 with one end connected to the outputting main line 7 and a termination grounded through a capacitor 9b, a drain biasing terminal 12 connected to a termination of the outputting stub 8 through a bias resistor 10 and an output terminal 14 connected to the outputting main line 7.
The above-described FETl is set to have a gate length of 0.25 .mu.m and a gate width of 200 .mu.m, which structure is common for use in the 12 GHz. In a case of using such as FET, in order to set the input/output impedance of the MMIC to 50.OMEGA., the characteristic impedances of the transmission lines 5, 6, 7 and 8 are set in the range from 30 to 120.OMEGA., requiring about 10.degree.-70.degree. of an electrical length .theta. (.theta.=2.pi.la/.lambda.) at 12 GHz. This length expressed in terms of a physical length l will be about 0.2-1.7 mm.
Since in an MMIC, a circuit for matching input and output impedances of an FET is ordinarily formed by distributed constant lines using microstrip lines or the like, lines of a length of the order of a millimeter are required at a frequency of about 10 GHz. Moreover, since the MMIC is formed on a semiconductor substrate of GaAs or the like, it is very difficult in terms of a chip size to form a long line of the order of a millimeter.
Now, shown in FIG. 13 is a conventionally reported example of a MMIC band-stop removing filter for use with SHF band direct satellite broadcast receiver. This filter comprises input and output terminals 15 and 16, main lines 18 and 19, and a stub 17 with one end connected to a point where the main lines 18 and 19 are connected and an open termination. The stub 17 is designed to have an electrical length .theta. (.theta.=2.pi.l/.lambda., expressed in terms of a physical length) which is 1/4 wavelength at the center frequency (fr) of a stop-band and a characteristic admittance Y17 to be 0.02 mho. Both of the characteristic admittances Y18 and Y19 of the main lines 18 and 19 are 0.02 mho.
The signal pass band of the SHF band direct satellite broadcast including those in the United States, Japan and Europe ranges from about 11.7 to 12.2 GHz and oscillator frequency is about 10.7 GHz, so that an intermediate frequency fr, of the image band will be about 9.4 GHz. In this case, if GaAs is used for a substrate of MMIC of FIG. 13, a physical length of about 2.8 mm is required for the stub 17. Now, shown in FIG. 14 is a calculation of the transmission loss in the above-described MMIC filter circuit, assuming that Y17= Y18=Y19=0.02 mho and the physical length of the stub is 2.8 mm wherein the terminals 15 and 16 are supposed to terminate at 50.OMEGA.. FIG. 14 shows that the transmission loss at the center frequency fs of 12GHz in the signal pass band is about 4dB, and a band wherein a signal removal ratio of -20dB can be obtained is at about 600 MHz.
However, in the case of an image band-stop filter circuit for use with the SHF band direct satellite broadcast receiver an MMIC is structured using conventionally proposed distributed constant lines, such as microstrip lines or the like and, a stub length becomes as long, as above, about 2.8 mm as, so that it is difficult to make the circuit as an MMIC. In addition, there is a problem that transmission loss in the signal pass band (11.7-12.2 MHz) is as great as about 4dB.